ya, that's right,, for instance,, the gunman at Dawson college stalked and did reckon there, (and also at 2 other uni in montreal) before, to see where he'd get the most victims... so ya there's preparation,
but the way it meant is that, it's out of impulse, for the day, or if they do. For instance, I'm sure that there people out there that has done all that preparation and that are ready to do other attacks like that. it's just the matter of waking up one day and go, today is the day, out of impulse.
and like you said,, for dirty or chemical weapon, you got the producing to to, that can leave time for you to think and go 'wait a min, what am i doing?'